The present invention relates to an oil level gauge for oil-filled electrical power equipment. More particularly, the invention relates to an oil gauge for the insulating or cooling oil for oil-filled electrical power equipment such as a power transformer, a choke or a tap changer and especially for the oil expansion vessel thereof into which oil flows during heating up of the electrical power apparatus and from which oil returns to the transformer or other electrical power apparatus as the oil contracts as the temperature drops.
A magnetic oil level indicator of the MTO series is distributed by the assignee of this application and is described in a brochure identified as IN 185/02de-0401/1000. The oil level indicator is manufactured by the assignee (MESSKO Albert Hauser GmbH and Co. KG. This oil level indicator has a transmitter portion and an indicator portion which can be separably interconnected. The sensor unit comprises an oil-type or sealed flange plate and is installed directly in the wall of the oil-containing vessel. A float arm transfers the rise and fall of the oil in the vessel and on which the float rides to a spindle or shaft. This spindle is connected by a magnetic clutch with a display spindle or shaft in the display portion of the unit and the latter spindle operates a pointer which rides over a dial face. The display unit can additionally be equipped with one or two microswitches or sensitive switches which output an electric signal upon reaching a minimum xe2x80x9cminxe2x80x9d or maximum xe2x80x9cmaxxe2x80x9d position. The display is equipped, therefore, with both an optical read-out of the instantaneous filling state by means of the pointer and scale and an electrical unit which can provide an input for a control system via the electrical signal output at certain filling states, for example by means of the sensitive switch.
Another level indicator is available which produces a continuous analog electric signal as a measure of the degree of filling. In this case, the indicator shaft in the display part of the unit is connected with a potentiometer. The measured state of filling is converted into an angular displacement of the shaft of the potentiometer which supplies an analog signal representing the liquid level. A drawback of this level indicator is that the contact in the potentiometer is a mechanical slide contact which rides upon a circular conductor track and in many instances the wiper contact and its conductive track contribute problems of wear, problems of contamination when, for example, the contacts are not relatively moved frequently and even problems of film formation and modified contact resistance when the system is exposed to aggressive media or even air for prolonged periods.
It is, therefore, the principal object of the present invention to provide an improved liquid level detector especially applicable to the measurement of the oil level of an oil-filled electrical power apparatus, whereby drawbacks of the earlier system are avoided.
Another object of the invention is to provide an electrical level detector which can be used in the oil expansion reservoir, of, for example, a power transformer, an electric power choke or an electric power tap changer which does not suffer change with time even though the use may be infrequent and which can provide a continuous measure of the oil level.
These objects and others which will become apparent hereinafter are attained, in accordance with the invention in an oil level indicator for the oil expansion vessel of an electric power transformer, choke or tap changer in which a float is operatively connected to a float rod and which converts the lifting movement of the float to the rotation of a shaft of an angle encoder whose input member is operatively connected with the shaft and preferably with a shaft carrying an indicator or pointer which rides on or over a scale. According to the invention the angle encoder is of an electromagnetic type having a magnetic hub which cooperates with a contactless pick-up in the form of a Hall element with integrated ASIC to form an ASIC Hall sensor, converting the position of the magnetic actuator into an absolute incremental or sine-cosine output signal.
According to a feature of the invention, the angle indicator is coupled with the shaft driven by the float with belt coupling, the shafts of the encoder and the float system having respective wheels around which the belt passes. The belt may be any conventional coupling belt or endless element such as a cog belt, a chain or other flexible member connecting the driven shaft and wheel with the driving shaft and wheel.
The system of the invention allows a miniature angle encoder to be used as the contactless angle sensor and which is free from wear and is completely free from the effects of contact film formation and contamination of mutually sliding surfaces. the miniature angle encoder can be any of those marketed under the designation RXM22, for example of the firm TWK Electronik GmbH, Dxc3xcsseldorf, Germany and described in the brochure xe2x80x9cELEKTROMAGNETISCHE WINKELCODIERER UND IMPULSGEBERxe2x80x9d for the models RBM of the same firm. Reference may be made to the brochure RXM10937BE* of April, 2002 obtainable from the web site of this company.
As far as we know, such encoders have never been utilized for electric power equipment and especially not in conjunction with float sensors for the oil level in an expansion tank of an oil-filled transformer, choke or tap changer although they appear to have been used for other purposes in conjunction with automotive technology. The unit has a flange and a housing of aluminum, a magnetic hub with north and south poles and journal structures of stainless steel to receive a shaft. The detection of the angular position is effected by means of a Hall sensor which is integrated together with the signal processor on an ASCIC and can be juxtaposed with the magnetic hub so that no moving part requires contact with a wiper or, conversely, has a wiper which must engage a stationary contact part.